Minimizing the Focal Shift Effect on 2D Metalenses

For a metalens, which obeys the fundamental laws governed by diffraction, improving the image quality and resolution at the focal region has been the most valuable and desirable feature [1]–[3]. Early classical investigations claim that the principal maximum intensity of the diffracted wave is located at the geometric focus, and the field exhibits definite symmetry about the geometrical focal plane. However, with the extended use of lasers it is shown that during the experiments the principal maximum intensity of the incident wave may not occur at the preset geometrical focus butt may be located closer to the lens. This phenomenon is referred as the ‘focal shift’. One important detrimental result of the focal shift phenomenon is the asymmetric intensity distribution in the focal region [4]–[6]. Thus, reducing the focal shift for a better performing metalens is critical as it leads to a symmetric airy disc like field distribution on focal spot.